The long-term goal of the research is to understand the mechanisms used by cells to impart specificity to signal transducers through compartmentalization and restricted localization. In particular we are interested in the Rab GTPase subfamily of the Ras superfamily. Rab GTPases are important signal transducers and essential elements of the membrane trafficking machinery. Like other Ras-related GTPases, Rab proteins are prenylated and exist in both a soluble pool and on the cytosolic surface of an organelle. A characteristic feature of Rab proteins is that each Rab is localized to a distinct subcellular membrane. Only once recruited to a membrane can Rab GTPases exert their regulatory function, through activation by guanine-nucleotide exchange factors (GEFs) and the subsequent recruitment of effector proteins. The mechanisms that serve to attach Rab proteins to membranes and regulate the activation event are presently unknown. Our studies will address this question, with a combination of several different approaches, including mutant analyses, genomic studies, biochemical, and cell biological techniques and will focus on the model organism S. cerevisiae. The proposed studies are divided into two specific aims. Specific Aim 1 involves studies of a Rab binding partner, Yip1p, that we feel represents an excellent candidate for playing a fundamental role in membrane localization of Rab proteins. In Specific Aim 2, we take a converse approach and examine the features of individual Rabs that confer specificity to a given organelle. Overall, we believe the combination of these two aims will help elucidate the long-standing problem of how Rab proteins become recruited onto specific organelles. Aim 1: Examine the role of Yip1p, an essential Rab-interacting membrane protein and its influence on Rab function. Aim 2: Identify and characterize the specificity determinants responsible for the restricted localization of Rab proteins. A central issue in cell biology has been to understand the mechanisms of membrane transport, and the pathophysiological significance of disrupted transport. Understanding how cells correctly localize and activate Rab GTPases will provide an important missing piece to the puzzle of how these proteins function and how abnormalities in these fundamental processes may contribute to, and be the cause of human disease.